Balanced and Unbalanced Balanced and Unbalanced ForcesForces

Velocity and AccelerationVelocity and Acceleration

Force and Force and MotionMotion

MotionMotion• What is motion?

– A change in the position of an object over time.

• How do you know something has moved?– You use a reference point!

• A stationary (not moving) object such as a tree, street sign, or a line on the road.

Did the beaver move?

•What causes an object to move?– A FORCE!– ALL motion is due to forces acting on

objects!

•What is a force?– A push or a pull

FORCE MOTION

Can more thanone force act onan object at the

same time?

#2

#3

#1

YES!Example: Gravity is

pulling you down to Earth, the ground is supporting you, and your legs moving you forward as you run during PE.

The total combination of the forces acting on an

object is called NET FORCE.

Balanced ForcesBalanced Forces• A balanced force is one in which

the net force equals ZERO.• Do you think there will be any

motion?– NO!

• Examples:

25 Newtons 25 Newtons

50N

50N

Unbalanced Unbalanced ForcesForces• An unbalanced force is one in

which the net force is greater than zero.

• Do you think there will be any motion?– YES!

• Examples:

25 Newtons 40 Newtons 2 N

50N

Only an _______________ Only an _______________ force can change the force can change the motion of an object.motion of an object.

• Example: Your doggy can cause you to move if he pulls with enough force.– His force is greater

than the force you’re using to stay in place

What would happen if an What would happen if an unbalanced force acted on unbalanced force acted on an object that’s already in an object that’s already in

motion?motion?• It will change the

speed or direction of the object.

• Example: Your little brother is riding his tricycle. You run up behind him and give him a push.– Your force adds to the

existing force causing him to speed up.

Unbalanced forces can Unbalanced forces can act in the act in the samesame

direction.direction.• Example: You’re pushing a cabinet across

the room with a force of 15 N. You’re friend is pulling with a force of 10 N.

• What is the NET FORCE?• What direction is the cabinet moving?

15 Newtons 10 Newtons

Unbalanced forces can Unbalanced forces can act in act in oppositeopposite

directions.directions.• Example: Two dogs are tugging on a

rope. One dog pulls with a force of 20N and the other pulls with a force of 25N.

• What is the NET FORCE?• What direction is the rope moving?

25 Newtons20 Newtons

When you have

opposing forces,

the direction the

object moves is in

the same direction

as the larger force.

Mass and InertiaMass and Inertia• Newton’s 1st Law:

The Law of Inertia– An object at rest will

remain at rest, unless acted upon by an unbalanced force

– An object in motion will continue moving, in the same direction, at the same speed, unless an unbalanced force acts upon it.

InertiaInertia• Inertia is the

tendency of objects to resist a change in motion.

• Example: seatbelts!– http://

www.youtube.com/watch?v=2DWFQ73cevU

50 mph

50 mph

MassMass• The mass of an object affects its’ inertia.• Objects with more mass have more inertia

than an object with a smaller mass.– It’s harder to make a large object move or

change the speed and direction of it when it’s moving.

Another ExampleAnother Example• Train v. Car: Which will take longer to

accelerate to 60 mph? Why?

Friction and Friction and GravityGravity• What is friction?

– A force that opposes the motion of an object

– It’s a “contact” force!• Occurs when an object in motion rubs

against a surface.• The contact reduces the speed of the object

and releases heat.

What affects the What affects the amount of friction?amount of friction?

• The force of the push/pull– The harder you push,

the longer it’s going to take friction to stop the object.

• The bumpiness of the surface– The rougher the

surface, the more friction.

• The weight of the object– The heavier the object,

the more friction.

• What is gravity?– The force of attraction between all

objects.

• The amount of gravity depends on two things:– The objects’ masses– The distance between the two objects

• Since the earth is so large, everything on it is attracted to it even if they’re not touching!

• Example: Throwing a ball.– You throw a ball up, but

gravity pulls it back down to earth.

– You can counteract gravity by catching the ball before it hits the ground (you provide the outside force!)

Penny Lab

• Material:Beaker, index card, penny.• Hypothesis: What will happen if you flick an

index card out with a penny on top?• Place index card over beaker, then penny on top

over the hole.• Flick card out. • Create 3 trials with data.• Write a conclusion using inertia, gravity,

reference point, motion, velocity, force, distance, and friction. Include how this applies to daily living.